I'm not exactly sure what the OP is referring to. I've never once seen an AR pistol with a 3" barrel. Most AR pistols I've seen have 10.5" barrels, and many others are 7.5".
The purpose of an AR pistol is to get around the SBR laws. If you use an AR for home defense, then the compactness of an AR pistol is nice, expecially if you have a silencer on it.
I have a 10.5" SBR with a silencer as my primary home defense weapon. Even with the silencer attached, it's still slightly shorter than a regular 16" AR. And the 10.5" barrel is just long enough that I still get pretty good effectiveness, especially if I use heavier bullets.
If this rifle was an AR pistol with an "arm brace" instead of an SBR, it could still serve the same purpose and be used in almost the exact same way.
Now, many other people use their AR pistols as just fun range toys, especially the people who have 7.5" barrels. And that's fine too. But AR pistols can be practical firearms depending on how they're used.